1. Field of the Invention
The present invention relates to a rare-earth magnet for which a protective film is provided over the magnet body containing rare earth elements.
2. Description of the Related Art
A well-known exemplary rare-earth magnets includes a Sm—Co5-based, Sm2—Co,17-based, Sm—Fe—N-based, or R—Fe—B-based magnets (R represents a rare earth element), which are used as a high-performance permanent magnet. Among them, the R—Fe—B-based magnet especially attracts a high attention because it includes as its main rare-earth element Neodymium (Nd), which can be obtained easily and at a comparatively low cost compared with Samarium (Sm), while iron (Fe) is also obtainable at low cost, and further, its magnetic performance is equivalent to or more excellent than that of the Sm—Co-based magnets.
However, since the R—Fe—B-based rare-earth magnet contains as its principal component a rare earth element and iron that are easily oxidizable, its corrosion resistance is comparatively low. Thereby, deterioration, dispersion, etc. of its practical performance have been a problem.
In order to improve the foregoing problem of low corrosion-resistance of such rare-earth magnets, formation of a protective film made of an oxidation-resistant metal etc. on the surface thereof has been proposed. For example, those coated by overlaying two metal-plating layers made of nickel (Ni) is disclosed in Patent Document of JP. Pat. No. 2599753 description, and those coated by overlaying a metal-plating layer made of nickel(Ni)-sulfur(S) alloy subsequent to overlaying a metal-plating layer made of nickel is disclosed in Patent Document of Japanese Laid-Open Patent Publication (Kokai) No. 106109/1995.